1. Field of the Invention
The present invention relates to a method of injection molding a polyethylene terephthalate in which a polyethylene terephthalate in an undried state can be injection molded into a required molded article, and even an irregular-shape molding material with irregular size such as flake, pellet and powder-like one can be fed to an injection machine.
2. Background Art
A polyethylene terephthalate (hereinafter abbreviated as PET) generally used as a material resin for packaging containers and the like has moisture-absorption characteristics, so that if a PET, as it is, is fed as a molding material to an injection machine, a poor biting into a screw, a variation in metering, or a hydrolysis due to the vaporization of the water content contained therewithin would occur, and thus a predrying has been considered to be required when using the PET.
Particularly, a molding material which is manufactured by pulverizing used PET products into flake-like pieces in order to reuse them has an irregular size of flake and a water content due to moisture absorption varying with respective flakes, so that the material requires a more effective predrying than an unused molding material having even particles.
The above-mentioned preheating requires about four hours at a temperature of 150.degree. C. for PET used as a crystal resin. Therefore, the predrying must be started in the early morning, and where an automatic molding is performed continuously around the clock, the setting of feed rate must be strictly performed so that drying is compatible with molding. An inconvenience also exists in that even when injection molding is successfully being performed, a trouble with a dryer causes molding to have to be stopped.
When a molding material is flake or powder in shape, an attempt to feed the material from a hopper to an injection machine by gravity causes the material to form a bridge in the hopper and thus feed rate to become unstable. As conventional means for preventing such phenomenon to effect a stable feed, there has been employed a hopper including a screw.
A method of disposing a feed screw in the material feed section of an injection molding or an extrusion molding machine, and forcedly feeding a molding material of a thermoplastic resin into the molding machine by the rotation control of the feed screw is described in Japanese Laid-Open Patent Publication Nos. 60-116424, 5-318531 and the like.
The means described in No. 60-116424 detects the pressure of a resin material at a lower portion of a screw type forced feeder disposed in an extrusion machine and at a screw base portion of the extrusion machine by the use of a sensor, and with the output of the sensor, controls the revolution speed of the forced feeder screw so as to make constant the pressure of the resin material at all times, whereby a powder material, as well as a mixed raw material of powder and pellet, a flake-like raw material and the like can be fed stably.
Also, the means described in No. 5-318531 sequentially detects a reaction force preventing the rotation of an injection screw, and controls increasingly/decreasingly the feed speed of a pellet feeder in correspondence with the increase/decrease in the reaction force, thereby controlling the pellet feed.
In the above-mentioned means of detecting the pressure of the resin raw material to control the revolution speed of the forced feeder screw, with the fed-out force due to the forced feeder screw, it is difficult to keep constant the pressure of a flake-like molding material having a density larger than a powder-like material, and the pressure is apt to change. Therefore, the rotation control is performed frequently, and even if a rotation control by the detection of the pressure of a resin raw material is employed, the stable feed of the flake-like molding material cannot be performed due to a problem with response.
The other above-mentioned means for sequentially detecting a reaction force preventing the rotation of an injection screw, and controlling increasingly/decreasingly the feed speed of a molding material is as with the above means, so that new means is required to attain a stable feed of a flake-like, irregular-shape molding material.